Tractors are often used in applications, for example loading work or cultivation work, where frequent changes between forward and reverse gears are required. To avoid repeated de-clutching the power shuttle gearbox was developed.
The power shuttle gearbox allows the driver of a vehicle, for example an agricultural tractor, to change the direction of travel from forward to reverse or vice versa simply by pressing a button or pulling a lever. It is not necessary for the driver to use the clutch pedal at all to initiate a change of direction gear change. Some power shuttle gearboxes also allow gearshifts from neutral to a forward gear, or neutral to a reverse gear to be executed.
One type of power shuttle gearbox requires the hydraulic actuation of the clutch. The performance of such a power shuttle gearbox is influenced by the time taken for the hydraulic actuator to move the clutch pack from its biting point to it fully engaged positions. This is known as clutch take up. Changing the duration of this period changes the aggressiveness of clutch take up, and therefore the aggressiveness with which drive is taken up.
Tractors generally have a plurality of gears and gear ranges. For example, a transmission may comprise a four-speed gearbox coupled to a three-speed range change, thereby giving twelve gear ratios. The provision of a power shuttle gives the transmission twelve forward and twelve reverse gear ratios.
It is known to program the power shuttle so that for each gear ratio the time taken for the clutch take up differs. Whilst this may provide a smooth transition during shuttling, a smooth transition is not always required.
One problem associated with the above-mentioned type of power shuttle is that a tractor may be required to do two different jobs using the same gear ratio, but the jobs may have different requirements for aggressiveness of clutch take up.
Another problem associated with the above-mentioned type of power shuttle is that different drivers have different personal requirements from a tractor. Whilst one driver may not mind aggressive clutch take up, another my find this uncomfortable.
It would therefore be desirable to provide a control system for a power shuttle gearbox which does not suffer from the problems associated with the above-mentioned prior art.